Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by unit structure of the crystal. The zeolites are referred to as "molecular sieves" because interconnecting channel systems created by pores of uniform pore size allow a zeolite to selectively absorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/n O.multidot.Al.sub.2 O.sub.3 .multidot.xSiO.sub.2 .multidot.yH.sub.2 O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated M. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p.5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which complete the electrovalence makeup of the empirical formula.
The crystal lattice framework of these aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing alumina is balanced by the inclusion in the crystal of a cation, for example an alkali metal, an alkaline earth metal or an organic cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The cavities and pores are occupied by molecules of water prior to dehydration and/or possibly by organic species from the synthesis mixture in the as-synthesized materials.
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite beta (U.S. Pat. No. 3,308,069 and Re 28341); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No., 3,314,752); zeolite ZSM-5 (U.S. Pat. No., 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979) and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The silicon/aluminum atomic ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with silicon/aluminum atomic ratios of from 1 to 1.5, while that ratio in zeolite Y is from 1.5 to 3. In some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is at least 2.5 and up to infinity. U.S. Pat. No. 3,941,871, reissued as RE. 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the X-ray diffraction pattern characteristic of ZSM-5 zeolites.
Moreover, the silicon/aluminum atomic ratio of the "as-synthesized" zeolite can be altered, specifically increased, by decreasing the tetrahedral alumina thereof. Decrease in the tetrahedral alumina may be effected by synthetic methods developed to deplete the tetrahedral alumina of a zeolite. In addition, the silicon:aluminum atomic ratio of a zeolite may be increased, that is there may be a loss of tetrahedral alumina, as a result of process conditions to which the zeolite may be subjected during use. Process conditions which will effect depletion of tetrahedral alumina include high temperature calcination and steaming. This loss of aluminum does not affect the crystallinity of zeolites, such as ZSM-5.
Those zeolites of practical significance today are not only characterized by uniform pore sizes, but also by channel systems created by those pores. To maintain activity of a zeolite, the crystallographic structure of the zeolite after chemical treatment must remain in tact.
Various chemical treatments of zeolites have been proposed to modify their chemical properties. In accordance with the invention, the chemical treatment described introduces additional hydroxy groups to the zeolite. In J. Datka et al "Hydroxyl groups and acid sites in Na-ZSM-5 zeolites studied by i.r. spectroscopy" Zeolites Vol. 5 No. 4, p. 230 (July 1985), non-acidic hydroxyl group presence in Na-ZSM-5 was attributed to the i.r. band at about 3738 cm.sup.-1 ; in addition, J. Datka et al referred to the i.r. work of Jacobs and Ballmoos who assigned OH groups vibrating at about 3720-3730 cm.sup.-1, to extra zeolitic material which might be introduced into Na-ZSM-5 during synthesis or during the decomposition of organic entities in Na-ZSM-5.